Melanin is a class of compouns found in the plant, animal and protista kingdoms, where it serves predominantly as a pigment. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole (also known as hydroquinone), dihydroxyindole carboxylic acid, and their reduced forms. Another common form of melanin is pheomelanin, a red-brown polymer of benzothiazine units largely responsible for red hair and freckles. The presence of melanin in the archaea and bacteria kingdoms is an issue of ongoing debate amongst researchers in the field. The increased production of melanin in human skin is called melanogenesis. It is stimulated by the DNA damages that are caused by UVB-radiation, and it leads to a delayed development of a tan. This melanogenesis-based tan takes more time to develop, but it is long lasting.